Automotive hydraulic control systems have been proposed in which the pressure of a control fluid, such as engine oil, is controlled for positioning of a hydraulic actuator. Control fluid viscosity can vary significantly with fluid temperature and age. Control fluid pressure can vary significantly during even one control cycle. Variations in fluid viscosity and pressure significantly affect dynamic hydraulic control performance. Accordingly, some attempt has been made to estimate control fluid viscosity and pressure and vary control gains in response thereto. For example, control fluid age, temperature and pressure have been measured or estimated and the temperature and age estimations used to estimate fluid viscosity, and the estimated viscosity and pressure used to vary control gains. Such complex sensing, estimating and processing yields some improvement in dynamic hydraulic control system performance.
However, control fluid temperature, age and pressure are only three of many factors that may affect dynamic hydraulic control performance. Furthermore, fluid temperature and pressure sampled at one point in a hydraulic system may not reflect accurately the temperature and pressure of the control fluid a short distance away or a short time later. Still further, the relationship between estimated or measured fluid age and change in fluid viscosity may be difficult to accurately characterize. Still further, use of temperature, age and pressure sensors or estimators adds to control system cost and complexity.